U.S. patent application number 11/085174 was filed with the patent office on 2006-05-04 for image forming apparatus and image forming method.
This patent application is currently assigned to FUJI XEROX CO., LTD.. Invention is credited to Osamu Goto, Takeshi Kato, Yoshiki Matsuzaki, Kozo Tagawa.
Application Number | 20060092264 11/085174 |
Document ID | / |
Family ID | 36261309 |
Filed Date | 2006-05-04 |
United States Patent
Application |
20060092264 |
Kind Code |
A1 |
Matsuzaki; Yoshiki ; et
al. |
May 4, 2006 |
Image forming apparatus and image forming method
Abstract
An image forming apparatus having a plurality of light sources
and printing an image carrier by collectively scanning an image
carrier with beams from the plurality of light sources includes a
screen processing unit for performing a screen process on input
image data and a registration correction processing unit for
performing skew correction on the image data on which the screen
process has been performed and for performing an image shift
process in the sub-scanning direction, which is a moving direction
of the image carrier, based on a periodic characteristic of
exposure by the collective scanning and the period of the screen by
the screen process.
Inventors: |
Matsuzaki; Yoshiki;
(Kanagawa, JP) ; Tagawa; Kozo; (Kanagawa, JP)
; Kato; Takeshi; (Kanagawa, JP) ; Goto; Osamu;
(Kanagawa, JP) |
Correspondence
Address: |
MORGAN LEWIS & BOCKIUS LLP
1111 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20004
US
|
Assignee: |
FUJI XEROX CO., LTD.
|
Family ID: |
36261309 |
Appl. No.: |
11/085174 |
Filed: |
March 22, 2005 |
Current U.S.
Class: |
347/233 |
Current CPC
Class: |
G03G 2215/0404 20130101;
G03G 15/0435 20130101; G03G 15/0194 20130101; G03G 2215/0119
20130101; G03G 15/326 20130101; G03G 2215/0158 20130101 |
Class at
Publication: |
347/233 |
International
Class: |
B41J 2/455 20060101
B41J002/455 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2004 |
JP |
P.2004-315682 |
Claims
1. An image forming apparatus comprising: an input unit for
inputting an image data; a printing unit having a plurality of
printing sources and collectively scanning an image carrier with
the printing sources or beams from the printing source to print the
image carrier; and an image shift processing unit for performing an
image shift in a sub-scanning direction, which is a moving
direction of the image carrier, based on a periodic characteristic
of printing by the printing unit and a periodic characteristic of
an image when the image data input by the input unit is drawn.
2. The image forming apparatus according to claim 1, wherein: the
plurality of printing sources of the printing unit is a plurality
of laser beam sources; and the printing unit is an exposure unit
using multi-beams that collectively irradiates a plurality of laser
beams from the plurality of laser beam sources using a rotary
polygon mirror.
3. The image forming apparatus according to claim 1, wherein the
periodic characteristic of printing used for the image shift
processing unit is a periodic printing disarrangement caused by the
effective number of lines collectively scanned by the printing
unit.
4. The image forming apparatus according to claim 1, wherein the
periodic characteristic of printing used for the image shift
processing unit is a characteristic caused by an arrangement shape
of the plurality of printing sources included in the printing
unit.
5. The image forming apparatus according to claim 1, wherein the
periodic characteristic of printing used for the image shift
processing unit is caused by a physical characteristic of the
plurality of printing sources included in the printing unit.
6. The image forming apparatus according to claim 5, wherein when
the printing unit is a multi-beam exposure unit, the periodic
characteristic caused by the physical characteristic is a period
variation of at least one of the exposure position, amount of a
light, and a diameter of a spot caused by an optical system
including a light source.
7. The image forming apparatus according to claim 1, wherein the
image shifting process by the image shift processing unit is at
least one of an image inserting or a thinning out process.
8. The image forming apparatus according to claim 1, wherein the
periodic characteristic of the image used for the image shift
processing unit is a characteristic caused by the shape and
position of a binary image.
9. An image forming method used for an image forming apparatus
having a plurality of printing sources and collectively scanning an
image carrier with the printing sources or beams from the printing
sources to print the image carrier, the image forming method
comprising: performing a screen process on an input image data;
performing a skew correction on the image data on which the screen
process has been performed; and performing an image shifting
process in a sub-scanning direction, which is a moving direction of
the image carrier, based on a periodic characteristic of printing
by the collective scanning and a screen period by the screen
process.
10. The image forming method according to claim 9, wherein: the
image forming apparatus can perform a color printing and has a
plurality of printing sources each corresponding to a different
color; in the screen process, a different screen is selected for
each color; and the image shifting process is performed when the
screen selected for each color synchronizes with the period of
printing performed by the collective scanning.
11. The image forming method according to claim 10, wherein when
the image shifting process is performed on at least one of a
plurality of colors, the image shifting process is also performed
on the other colors.
12. The image forming method according to claim 9, wherein the skew
correction shifts the output image data stepwise in the main
scanning direction in which the collective scanning is performed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an image forming apparatus,
such as a printer or a duplicating machine, and particularly, to an
image forming apparatus capable of printing a color image or a
black-and-white image.
[0003] 2. Background Art
[0004] In recent years, image forming apparatuses capable of
forming a color image, such as a printer and a duplicating machine,
have been widely spread. As this type of general image forming
apparatus, there has been used a so-called tandem-type image
forming apparatus in which image forming units respectively
provided correspondingly to, for example, black (K), yellow (Y),
magenta (M), and cyan (C) are arranged in parallel to a transfer
object (a transfer belt, serving as an intermediate transfer
member, and a sheet of paper, which is a recording medium). In this
tandem-type image forming apparatus, images, having different
colors, formed by the respective image forming units are
sequentially transferred to a running transfer object and are then
multiplexed, thereby forming a color image.
[0005] In this tandem-type image forming apparatus, the images
having different colors overlap each other to form a color image.
Therefore, color misalignment may occur in the formed image by
deviation of mounting position of each image forming unit, an error
in the peripheral velocity of each image forming unit, deviation of
exposure position with respect to a transfer object, variation of
the linear velocity of the transfer object, etc. That is, the
alignment and mechanical errors of the image forming units provided
correspondingly to each color leads to color misalignment on a
recording medium (for example, a sheet) as they are. Further, in
this type of image forming apparatus, it is indispensable to
measure the amount of the color misalignment and to perform color
misalignment control (registration control) to prevent the color
misalignment. As a general method of controlling the color
misalignment, there is a method of drawing yellow, magenta, cyan
and black marks (patterns) on a transfer target, of reading the
positions of the marks using a sensor, of calculating the amount of
color misalignment from a result read, and of feeding it back to
control the image forming units. Further, in addition to the
tandem-type image forming apparatus, for example, a cycle-type
image forming apparatus of forming a color image by rotating an
image carrier plural times and a so-called inkjet-type image
forming apparatus also have the same problems as described
above.
[0006] As a conventional technique disclosed in patent documents,
there has been disclosed a technique in which registration marks
are formed at two different places on a transfer belt in the main
scanning direction, the amount of color misalignment between a
reference color and the other colors is calculated, an approximate
function for correction is calculated based on the amount of the
color misalignment, and an address of an image is changed to
perform skew correction (for example, see JP-A-2000-112206).
Further, there also has been disclosed another technique of
correcting skew generated in printer head in a line unit by
changing a writing address of an image and outputting it with a
step difference (for example, see JP-A-2001-80124).
[0007] Furthermore, in recent years, there has been proposed an
image forming apparatus using a so-called multi-beam scanning
optical system of collectively irradiating a plurality of laser
beams from a plurality of light sources by a rotary polygon mirror.
Moreover, there has been proposed a technique in which, in an image
forming apparatus using a multi-beam scanning optical system,
sub-scanning misalignment is corrected by selecting one light
source for performing recording on a first line from a plurality of
light sources according to a difference in time between an image
recording start signal and a laser operation synchronizing signal
(for example, see JP-A-8-142412).
[0008] FIGS. 13A and 13B are views illustrating an example of a
conventional process of correcting the misalignment of the
sub-scanning direction in the image forming apparatus using the
multi-beam scanning optical system (multi-beam raster output
scanner (ROS)). In FIGS. 13A and 13B, the horizontal direction
indicates a main scanning direction (A to R), and the vertical
direction indicates a sub-scanning direction (1 to 24). In
addition, scanning is performed using four multi-beams. As shown in
FIG. 13B, the output position is changed in order to correct the
misalignment of the sub-scanning direction, and the change is
performed by shifting a laser used for forming an image by the
multi-beams.
[0009] Further, FIGS. 14A and 14B are views illustrating a
conventional process of performing skew correction in the image
forming apparatus using the multi-beam scanning optical system.
Similarly in FIGS. 13A and 13B, in FIGS. 14A and 14B, the
horizontal direction indicates the main scanning direction, and the
vertical direction indicates the sub-scanning direction. In
addition, scanning is performed using four multi-beams. In FIG.
14B, output image data is stepwise shifted in the main scanning
direction with respect to FIG. 14A. In this way, it is possible to
make skew distortion smaller than that in a general line. For
example, it is possible to perform good skew correction on colors
drawn to be inclined with respect to the reference color.
[0010] Furthermore, FIGS. 15A to 15F illustrating the shift of a
half-tone image in correcting the misalignment of the sub-scanning
direction in the image forming apparatus using the multi-beam
scanning optical system. The multi-beam ROS shown in this example
is composed of four beams (LD1 to LD4) as shown in FIG. 15F, and in
order to output an image having a width corresponding to 16 lines
in the sub-scanning direction as shown in FIG. 15A, it is necessary
to perform scanning four times as shown on the left side of FIG.
15A. FIGS. 15A to 15E illustrate the shift of a predetermined
half-tone image (a check composed of a white-and-black binary image
in a matrix of four by four dots) in the sub-scanning direction. In
the image shown in FIG. 15A, writing starts using the laser LD1
shown in FIG. 15F, and in the image shown in FIG. 15B, writing
starts using the laser LD2. Further, in the image shown in FIG.
15C, writing starts using the laser LD3 shown in FIG. 15F, and in
the image shown in FIG. 15D, writing starts using the laser LD4.
Furthermore, in FIG. 15E, writing is not performed by first
scanning, but performed using the laser LD1 at the second scanning
timing. In the example shown in FIGS. 15A to 15F, the gaps between
the four beams shown in FIG. 15F are equal to each other, so that a
half-tone image is not disarranged on a sheet as shown on the lower
side of each of FIGS. 15A to 15E. That is, even when the skew
correction or the correction of misalignment in the sub-scanning
direction is performed, a substantially ideal image is
obtained.
[0011] However, when misalignment occurs in the gap between the
beams in the multi-beam scanning optical system, a preferred image
is not obtained.
[0012] FIGS. 16A to 16F illustrate an example of the shift of a
half-tone image when misalignment occurs in one laser in correcting
the misalignment of the sub-scanning direction in the image forming
apparatus using the multi-beam scanning optical system. In FIGS.
16A to 16F, the same multi-beam ROS as that in FIGS. 15A to 15F is
used. However, as shown in FIG. 16F, the example shown in FIGS. 16A
to 16F are different from that in FIGS. 15A to 15F in that the
laser LD3 of the four lasers is misaligned to lean to the laser
LD2. FIGS. 16A to 16E illustrate the output of an image in a case
in which the four lasers LD1 to LD4 are shifted to correct the
misalignment of the sub-scanning direction. In this case, since the
laser LD3 is misaligned with respect to three other lasers LDs, a
gap is generated between the lasers LD3 and LD4. The position of
this gap with respect to the image shown in each of FIGS. 16A to
16E is changed (moved) on the drawn image by the shift of the
lasers LDs. That is, the output direction of striation is
misaligned by an image. Therefore, the variation of the gap
position appears as the variation of density as shown on the lower
side of each of the images in this example. This variation of
density is related to the shape and periodicity of an image to be
drawn and the scanning period of multi-beams. More specifically, in
the four-line periodicity of an image shown in FIG. 16A, four beams
of the ROS shown in FIG. 16F synchronize with each other and have
four-line (pixel) periodicity. Thus, a defect in image quality
occurs, and density irregularity also occurs. FIGS. 16A to 16F
illustrate a defect caused by the density variation generated on
the entire surface of a sheet of paper when correcting the
misalignment of the sub-scanning direction, which is generated when
the image writing position of the sub-scanning direction is varied
before and after color registration correction. That is, FIGS. 16A
to 16F illustrate the variation of density for each sheet
outputted.
[0013] FIG. 17 illustrates the variation of density on a sheet when
skew correction is performed by stepwise shifting output image data
in the main scanning direction. FIG. 17 shows a case in which the
skew correction as shown in FIGS. 14A and 14B are performed by the
combination of images shown in FIGS. 14A and 14B, under a state in
which the misalignment of the laser LD3 occurs. In FIG. 17, the
same density variation as that in FIGS. 16A to 16F are periodically
(in the order of A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E (A))
generated whenever the shift of skew is performed. Since the
density variation at the time of skew correction is a density
irregularity varied in the main scanning direction on a sheet, the
density variation at the time of skew correction has a greater
influence on a defect in image quality than the density variation
for each sheet.
SUMMARY OF THE INVENTION
[0014] The present invention is designed to solve the
above-mentioned problems, and it is an object of the present
invention to prevent a defect in image quality when the correction
of the sub-scanning direction or skew correction is performed, for
example, in an inkjet-type image forming apparatus having a
plurality of printing sources or in an image forming apparatus
using a multi-beam scanning optical system.
[0015] It is another object of the present invention to perform a
preferred image shifting process according to a periodic
characteristic of collective scanning and a periodic characteristic
of an image.
[0016] Therefore, according to the present invention, in order to
achieve the above objects, when an optical system of a multi-laser
ROS is used, it is determined whether to insert an image
additionally using a periodic characteristic of exposure by the
multi-laser ROS and a periodic characteristic of image data. That
is, an image forming apparatus according to the present invention
includes an input unit for inputting image data; a printing unit
having a plurality of printing sources and collectively scanning an
image carrier with the printing sources and beams from the printing
source to print the image carrier; an image shift processing unit
for performing an image shift in a sub-scanning direction, which is
a moving direction of the image carrier, based on a periodic
characteristic of printing by the printing unit and a periodic
characteristic of an image when image data input by the input unit
is drawn. Scanning by the printing sources may include scanning by
nozzles in an inkjet method. In this case, the image carrier
corresponds to a sheet of paper. Further, the term `printing` is
not limited to forming characters, such as text, but is used for
forming various images other than characters in a wide meaning,
that is, includes an exposure function in an image forming
apparatus adopting an electrophotographic manner, an ink
discharging function from a printer head in an image forming
apparatus adopting an inkjet manner, etc.
[0017] In the image forming apparatus according to present
invention, the plurality of printing sources of the printing unit
is a plurality of laser beam sources, and the printing unit is an
exposure unit using multi-beam that collectively irradiates a
plurality of laser beams from the plurality of laser beam sources
using a rotary polygon mirror.
[0018] Further, in the image forming apparatus according to the
present invention, the periodic characteristic of printing used for
the image shift processing unit is periodic printing disarrangement
caused by the effective number of lines collectively scanned by the
printing unit. For example, when adjacent exposure is performed
using 32 beams, the effective number of lines collectively scanned
is 32. In addition, in a case of double exposure, the effective
number of lines is 16.
[0019] Furthermore, in the image forming apparatus according to the
present invention, the periodic characteristic of printing used for
the image shift processing unit is a characteristic caused by the
arrangement shape of the plurality of printing sources included in
the printing unit. For example, when the plurality of light sources
are arranged in a matrix of M by N (where M and N are integral
numbers equal to or greater than 1), a periodic characteristic
caused by the number of M or N appears in the multi-beam exposure
unit.
[0020] Moreover, in the image forming apparatus according to the
present invention, the periodic characteristic of printing used for
the image shift processing unit is a characteristic caused by a
physical characteristic of the plurality of printing sources
included in the printing unit. For example, when the printing unit
is a multi-beam exposure unit, the periodic characteristic caused
by the physical characteristic is a period variation of at least
one of the exposure position, amount of light, and diameter of a
spot caused by an optical system including a light source.
[0021] Further, in the image forming apparatus according to the
present invention, the image shifting process by the image shift
processing unit is an image inserting and/or thinning out process.
In this case, it is possible to generate the plural variations of
density in the sub-scanning direction. As a result, it is possible
to make the periodicity of the main scanning direction
inconspicuous.
[0022] Furthermore, in the image forming apparatus according to the
present invention, the periodic characteristic of the image used
for the image shift processing unit is a characteristic caused by
the shape and position of a binary image.
[0023] Meanwhile, the present invention provides an image forming
method used for an image forming apparatus having a plurality of
printing sources and collectively scanning an image carrier with
the printing sources or beams from the printing sources to print
the image carrier. The image forming method includes a step of
performing a screen process on input image data; a step of
performing skew correction on the image data on which the screen
process has been performed; and a step of performing an image
shifting process in a sub-scanning direction, which is a moving
direction of the image carrier, based on a periodic characteristic
of printing by the collective scanning and a screen period by the
screen process.
[0024] Further, in the image forming method according to the
present invention, the image forming apparatus can perform color
printing and has a plurality of printing sources each corresponding
to a different color. In the screen process, a different screen is
selected for each color, and the image shifting process is
performed when the screen selected for each color synchronizes with
the period of printing performed by the collective scanning.
[0025] Furthermore, in the image forming method according to the
present invention, when the image shifting process is performed on
at least one of a plurality of colors, the image shifting process
is also performed on the other colors. In this case, even when
synchronization is not made on a specific color, it is possible to
prevent the generation of color misalignment by performing the
image shifting process.
[0026] Moreover, in the image forming method according to the
present invention, the skew correction stepwise shifts the output
image data in the main scanning direction in which the collective
scanning is performed.
[0027] According to the present invention having the
above-mentioned configuration, it is possible to realize a
preferred image shifting process according to a periodic
characteristic of collective scanning and a periodic characteristic
of an image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] These and other objects and advantages of this invention
will become more fully apparent from the following detailed
description taken with the accompanying drawings in which:
[0029] FIG. 1 is a view illustrating an image forming apparatus
according to an embodiment of the present invention;
[0030] FIG. 2 is a view illustrating an example of a laser device
used for an exposure device;
[0031] FIG. 3 is a view illustrating an ideal output state of a
surface emitting laser device;
[0032] FIGS. 4A and 4B are views illustrating defects in a
multi-beam ROS;
[0033] FIG. 5 is a view illustrating density irregularity when skew
correction is performed under a state in which the positions of the
laser diodes deviate;
[0034] FIGS. 6A and 6B are views illustrating a first example of a
defective image quality correcting method according to the present
invention;
[0035] FIGS. 7A and 7B are views illustrating density irregularity
in the first example;
[0036] FIGS. 8A and 8B are views illustrating a second example of
the defective image quality correcting method according to the
present invention;
[0037] FIGS. 9A and 9B are views illustrating density irregularity
in the second example;
[0038] FIG. 10 is a block diagram illustrating a structure of a
control unit for performing a defective image quality correcting
process;
[0039] FIG. 11 is a flow chart illustrating a process performed by
the control unit shown in FIG. 10;
[0040] FIGS. 12A and 12B are views illustrating a screen requiring
a process according to the present invention and a screen not
requiring the process, respectively;
[0041] FIGS. 13A and 13B are views illustrating an example of a
conventional method of correcting the misalignment of the
sub-scanning direction in an image forming apparatus using a
multi-beam scanning optical system;
[0042] FIGS. 14A and 14B are views illustrating an example of a
conventional method of correcting skew in the image forming
apparatus using the multi-beam scanning optical system;
[0043] FIGS. 15A to 15F are views illustrating an example of the
image shift of a half-tone image in correcting the misalignment of
the sub-scanning direction in the image forming apparatus using the
multi-beam scanning optical system;
[0044] FIGS. 16A to 16F are views illustrating an example of the
image shift of the half-tone image when positional deviation occurs
in a laser in correcting the misalignment of the sub-scanning
direction in the image forming apparatus using the multi-beam
scanning optical system; and
[0045] FIG. 17 is a view illustrating the variation of density on a
sheet of paper when skew correction is performed by stepwise
shifting output image data in the main scanning direction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] Hereinafter, preferred embodiments (hereinafter, referred to
as embodiments) of the present invention will be described in
detail with reference to the accompanying drawings.
[0047] FIG. 1 is a view illustrating an image forming apparatus
according to an embodiment of the present invention. The image
forming apparatus is a so-called tandem-type digital color
electrophotographic method. As shown in FIG. 1, the image forming
apparatus includes image forming units 10, exposure devices 13 each
of which forms an electrostatic latent image on a photosensitive
drum 11 of the image forming unit 10 as a printing function, and a
transfer belt 21, serving as an intermediate transfer member, for
carrying a toner image carried by the photosensitive drum 11. The
image forming units 10 are provided to correspond to yellow (Y),
magenta (M), cyan (C), and black (K). Hereinafter, when it is
necessary to discriminate these image forming units, the image
forming units are referred to as image forming units 10Y, 10M, 10C,
and 10K, respectively. However, when it is unnecessary to
discriminate these image forming units, the image forming units are
simply referred to as an image forming unit 10. In addition,
primary transfer rollers 23 for carrying an image on the transfer
belt 21 are provided at positions opposite to the photosensitive
drums 11 of the respective image forming units 10 on the inner side
of the transfer roller 21. Further, secondary transfer rollers 24
are provided at so-called secondary transfer positions where a
toner image carried by the transfer belt 21 is transferred onto a
sheet of paper, and counter rollers 25 are provided opposite to the
secondary transfer rollers 24 on the inner side of the transfer
belt 21. Furthermore, the image forming apparatus further includes
a sheet feeding cassette 27 for containing sheets, serving as
recording media, and a fixer 28 for fixing a transferred sheet. In
addition, the image forming apparatus still further includes a
control unit 31 for controlling a pixel inserting/thinning out
process and an image shift process for correcting color
misregistration and a color registration sensor 32 for reading a
pattern for controlling color misregistration formed in a
predetermined region of the transfer belt 21.
[0048] The control unit 31 generates digital image signals of an
image obtained by an image reading device (IIT) or image signals of
a pattern image used for controlling color misregistration, and
supplies them to the exposure device 13 to perform a writing
process for the transfer belt 21. The control unit 31 acquires the
detection result of the pattern for controlling color
misregistration from the color registration sensor 32 and analyzes
the amount of color misregistration based on the acquired
information to perform a necessary correction process. These
functions performed by the control unit 31 are executed by, for
example, a CPU (Central Processing Unit) that is
program-controlled. In addition, the control unit 31 includes a
non-volatile ROM (Read Only Memory) and a read/write RAM (Random
Access Memory). This ROM is stored with software programs for
controlling an image forming operation, a color misregistration
detecting and correcting operation, etc., executed by the control
unit and image information of the pattern for controlling color
misregistration. The RAM is stored with various information items
acquired according to the operation of the image forming apparatus,
such as various counter values, the execution number of jobs, and
execution information (time information) on the previous color
misregistration detecting process.
[0049] For example, digital image signals acquired by the image
input terminal (IIT) or an external personal computer (PC) and then
converted by an image processing device (not shown) are supplied to
the exposure devices 13 corresponding to the respective colors
through the control unit 31. The color registration sensor 32 is a
reflective sensor that forms a pattern (a ladder-shaped toner patch
and a chevron patch) for controlling color misregistration formed
on the transfer belt 21 on a detector composed of a photo diode
(PD) and then detects pulses when a central line of the patch
coincides with a central line of the detector. In order to detect
the relative color misregistration of the pattern for controlling
color misregistration by the patch formed on each image forming
unit 10, two color registration sensors 32 are provided along the
main scanning direction at, for example, a downstream side of the
image forming unit 10K arranged at the lowermost downstream side of
FIG. 1. In addition, a light emitting portion of the color
registration sensor 32 is provided with, for example, two infrared
LEDs (which emits light having a wavelength of 880 nm), so that
stabilized pulse output is secured. Therefore, the amount of light
emitted from the two LEDs can be adjusted (for example, in
two-stage manner).
[0050] In each of the four-color image forming units 10Y, 10M, 10C,
and 10K, various units for forming an image are provided in the
vicinity of the photosensitive drum 11 serving as an image carrier.
That is, various devices, such as an electrifying device for
electrifying the photosensitive drum 11, a developing device for
developing a toner image on the photosensitive drum 11 exposed by
the exposure device 13, and a cleaner for removing the remaining
toner from the photosensitive drum 11 after the toner image is
transferred onto the transfer belt 21, are provided around the
photosensitive drum 11. However, it is also possible that a
specific color image forming unit corresponding to a specific color
other than the general colors Y, M, C, and K, such as a corporate
color, which has not been used for forming a general color image is
provided as the image forming unit 10. In addition, it is possible
to use five or more colors including dark yellow other than the
four colors Y, M, C, and K as the general colors. However, in the
present embodiment, the axial direction of the photosensitive drum
11 serving as an image carrier is the main scanning direction, and
the moving direction by the rotation of the photosensitive drum 11
is a sub-scanning direction.
[0051] Here, in the exposure devices 13 for exposing the respective
photosensitive drum 11 of the four-color image forming units 10Y,
10M, 10C, and 10K, a multi-beam raster output scanner (ROS) is
used, and each exposure device has a plurality of light sources
composed of a plurality of laser diodes (LDs). Laser beams
irradiated from the plurality of light sources are collimated by a
collimating lens and then are scanned by a deflecting reflection
surface of a rotary polygon mirror. Then, the photosensitive drum
11 is scanned and exposed by a laser spot concentrated by an image
forming lens. The photosensitive drum 11 is rotatably driven by a
driving unit and is then exposed in a direction (the sub-scanning
direction) orthogonal to the laser scanning direction (the main
scanning direction), thereby realizing two-dimensional exposure
recording.
[0052] As the transfer belt 21, an endless belt formed by shaping a
synthetic resin film made of, for example, polyimide having
flexibility in a stripe shape and by connecting both ends thereof
using a welding unit is used. The transfer belt 21 is formed in a
rope shape, and at least a portion of the transfer belt 21 is
extended substantially in a straight line by a driving roller and a
backup roller. In addition, the four-color image forming units 10Y,
10M, 10C, and 10K and the primary transfer rollers 23 opposite
thereto are arranged substantially in the horizontal direction with
respect to the substantially straight line portion at predetermined
gaps. In the example shown in FIG. 1, the yellow image forming unit
10Y, the magenta image forming unit 10M, the cyan image forming
unit 10C, and the black image forming unit 10K are sequentially
arranged from the upstream side toward the downstream side of the
transfer process in the moving direction of the transfer belt 21.
In the transfer belt 21, the respective color images formed by the
image forming units 10 sequentially overlap each other by the
operation of the belt, thereby forming a color toner image.
Further, the color toner image formed on the transfer belt 21 is
transferred onto a sheet at a position including the secondary
transfer roller 24 and the counter roller 25 at the timing when the
movement of the transfer belt 21 corresponds to the carrying of the
sheet. Then, the sheet having the color toner image thereon is
carried to the fixer 28, and the color toner image is fixed on the
sheet by the fixer 28. Subsequently, the sheet is discharged to a
discharging tray provided at the outside of a case of the image
forming apparatus.
[0053] FIG. 2 is a view illustrating an example of a laser device
used for the exposure device 13. In the present embodiment, a
surface emitting laser device 40 as shown in FIG. 2 is provided in
the exposure device 13. The surface emitting laser device 40 is
provided with thirty-two laser diodes (LD1 to LD 32) 41 arranged in
a matrix of four by eight (arrangement shape) serving as printing
sources. Therefore, it is possible to simultaneously scan
thirty-two lines by thirty-two laser beams irradiated from the
thirty-two laser diodes 41. Since the thirty-two multi-beams are
irradiated from one device, the positional relationship between
thirty-two laser beams can be maintained to a certain degree of
precision. However, the positional deviation in the rotation
direction or the deviation of a scanning position can occur by
loose mounting of the device or temperature variation.
[0054] A problem of the scanning position misalignment will be
described with reference to FIGS. 3 and 4.
[0055] FIG. 3 is a view for explaining an ideal output state of the
surface emitting laser device 10. In FIG. 3, exposure is performed
in a state in which the surface emitting laser device 40 is not
misaligned, and first scanning and second scanning are performed
without misalignment, thereby obtaining a clear ladder pattern.
[0056] Meanwhile, FIGS. 4A and 4B show defects in the multi-beam
ROS. FIG. 4A shows a case in which exposure is performed by the
surface emitting laser device 40 at a position inclined from a
normal position (the position shown in FIG. 3) represented by a
dotted line. As shown in FIG. 4A, when the exposure position of the
surface emitting laser device 40 deviates in a counter clockwise
direction with the laser diode (LD1) 41 used as a fulcrum, the
deviation of the scanning position occurs in the sub-scanning
direction as shown in 4-A to 4-F of FIGS. 4A and 4B. In this case,
the larger the distance from the LD1 serving as a fulcrum becomes,
the greater the positional deviation is. Referring to the numbers
of the laser diodes 41 shown in FIG. 2, for example, LD 4 has
positional deviation greater than LD 5. Further, when the step of
the sub-scanning direction is changed as between LD4 and LD5 or
between LD8 and LD9, the gap between the scanning lines is wide. In
addition, at the scanning position of LD32, the greatest positional
deviation occurs between the sub-scanning direction and the main
scanning direction. As a result, the positional deviation between
LD32 and LD1 to be subsequently scanned becomes great. FIGS. 4A and
4B shows a small amount of positional deviation with emphasis, but
LD32 actually deviates by about several microns. When a laser
having a resolution of 2400 dpi is used, the positional deviation
of a maximum of 10.6 microns occurs.
[0057] In a state in which the positional deviation shown in FIGS.
4A and 4B occurs, when the surface emitting laser device 40 is
shifted so as to output an image in the sub-scanning direction, a
defect (in this case, gap) in the quality of an image occurs, which
results in generating the irregularity of color density.
[0058] FIG. 5 is a view illustrating the irregularity of color
density when skew adjustment is performed under a state the
positional deviation of the laser diode 41 occurs as shown in FIGS.
4A and 4B. In the example shown in FIG. 5, the irregularity of
color density occurs by deviation of the scanning position of the
laser diode 41 and synchronization of the image periods of four
pixels. In addition, the same color density periodically exists at
a four sub-scanning LD shift period by skew correction.
[0059] Further, in the above-mentioned example, the deviation of
the scanning position of the laser diode 41 has been described, but
the cause of the image quality irregularity is not limited to the
deviation of the scanning position. For example, the amount of
light emitted from the laser diode 41 (including a defective laser
diode 41 emitting no light) or the diameter of a spot has an
influence on the irregularity of color density, similarly to the
above, which causes a defect in image quality. Here, when the
periodicity of a badness in laser diode 41 (LD badness) is
completely asynchronous with the period of an image to be drawn, a
defect, such as the irregularity of color density, does not occur.
Therefore, it is difficult to accurately grasp the periodicity of
the LD badness. Accordingly, in the present embodiment, pixels are
inserted into predetermined positions corresponding to the original
image, and a portion of the original image is shifted in the
sub-scanning direction, so that the deterioration of image quality
caused by the periodic variation of the exposure position by the
multi-beams and by the periodicity of an image to be drawn is
reduced.
[0060] FIGS. 6A and 6B are views illustrating a first embodiment of
a bad image quality correcting method according to the present
embodiment. FIG. 6A shows an original image, and FIG. 6B shows a
shifted image (a corrected image). In the shifted image shown in
FIG. 6B, pixels are inserted into predetermined positions
corresponding to the original image, and then the image is shifted
in the sub-scanning direction. This process causes the relationship
between image data and an exposure LD before the insertion is
performed to differ from that after the insertion is performed.
More specifically, the exposure position shown in FIGS. 4A and 4B
varies, and in a case of the output image data shown in FIGS. 4A
and 4B, the relationship between the image data and the exposure LD
before the pixel insertion is performed turns to the density state
shown in 4-A of FIG. 4B. Meanwhile, as shown in FIG. 6B, when a
pixel is inserted in every row in the main scanning direction, the
image exposed after the pixel is inserted is shifted in the
sub-scanning direction by one pixel. As a result, the image having
the density shown in 4-A of FIG. 4B turns to the image having the
density shown in 4-B by the pixel insertion.
[0061] FIGS. 7A and 7B are views for explaining the density
irregularity of the first example. FIG. 7A shows a case in which
the correction is not performed, and FIG. 7B shows an aspect of
density irregularity when pixels are inserted at predetermined
gaps. FIGS. 7A and 7B show macro images and density irregularity,
respectively. In addition, FIG. 7B also shows a micro image. As
shown in FIG. 7A, in the image before correction shown in FIG. 6A,
the variation of density extending in the vertical direction is in
a vertical stripe, similarly to FIG. 5, and is perceived as density
irregularity in the macro image. Meanwhile, when the pixel
insertion is performed as in the first example of the present
embodiment, the variation of density occurs at a plurality of
positions (in this case, four) in the longitudinal direction (the
sub-scanning direction) by image shift as shown in FIG. 7B, and
periodicity in the lengthwise direction (the main scanning
direction) is not perceived. In the example shown in FIG. 7B, when
pixel insertion is performed in the longitudinal direction, for
example, at 500-pixel intervals at 2400 dpi, the variation of
density at 500-pixel intervals occurs. Thus, it is difficult to
visually observe density irregularity from a macro point of
view.
[0062] Further, in the first example, the pixels are inserted in
every row. However, when the pixels are inserted in such simple
arrangement, the interference between the inserted pixels and the
image data characteristics (a screen shape, etc.) may cause a
stripe-shaped defect in image quality. In order to make up for the
above-mentioned defect, the following second example is
effective.
[0063] FIGS. 8A and 8B are views illustrating the second example of
a bad image quality correcting method according to the present
embodiment. FIG. 8A shows the original image, and FIG. 8B shows a
shifted image (a corrected image). In the first example shown in
FIG. 6B, the pixels are inserted in every row. However, in the
second example shown in FIG. 8B, the insertion position deviates
from the sub-scanning direction and is set in a straight line at a
predetermined angle. In this way, it is possible to prevent the
generation of the stripe-shaped defect in image quality caused by
the interference between the position of the inserted pixel and the
image data characteristics. In addition, the same data as image
data on the position of an object to be inserted is inserted as the
inserted image data.
[0064] Further, various methods of specifying a pixel insertion
position are considered. For example, it is considered a method of
randomly setting the pixel insertion position in the width of a
certain sub-scanning line. In addition, there is a method of
setting the position in arrangement having various angles and
periodic components by the calculation of functions. Further, there
is a method of setting the position to correspond to data of the
original image. For example, it is possible to set the pixel
insertion position to deviates by an angle of 45.degree., or it is
possible to set to be asynchronous with the period of data of the
original image. Further, it is preferable that the pixel data to be
inserted be determined such that the density of the original image
can be maintained and that be inserted at the same rate as the
density of the original image. Furthermore, there is also a method
in which the pixel data to be inserted is determined by peripheral
pixel data. Moreover, it is effective that the same number of
pixels is inserted in every row.
[0065] FIGS. 9A and 9B are views for explaining the irregularity of
density in the second example. FIG. 9A shows a case in which
correction is not performed, and FIG. 9B shows the irregularity of
density when an insertion process is performed at a gap. FIGS. 9A
and 9B show the macro image and the irregularity of density,
respectively. In addition, FIG. 9B also shows a micro image. In the
image before correction shown in FIG. 9A, the variation of density
extending in the vertical direction is in a vertical stripe,
similarly to FIG. 5, and is perceived as density irregularity in
the macro image. Meanwhile, when pixel insertion is performed as in
the second example of the present embodiment, the variation of
density occurs at a plurality of positions (in this case, four) in
the longitudinal direction by image shift as shown in FIG. 9B, and
periodicity in the lengthwise direction is not perceived. Thus, it
is difficult to visually observe density irregularity from a macro
point of view.
[0066] As described above, in the first and second examples, a
portion of the original image is shifted in the sub-scanning
direction by inserting pixels into the original image having a
defect in quality. That is, phases are changed before and after the
pixels are inserted, and the laser diodes 41 used are changed. In
this way, the variation of density occurs in the sub-scanning
direction as well as the main scanning direction, which makes it
possible to reduce the irregularity of density. Meanwhile, the
pixel insertion process causes an increase in the number of image
data lines in the sub-scanning direction, which results in the
variation of magnification in the sub-scanning direction. That is,
when an insertion gap is, for example, 500 lines, an image
magnification of 0.2%(= 1/500) is made. In this case, such a degree
of magnification does not matter in the image used for general
business. However, in a case of images used for the commercial
purpose, such a degree of magnification may raise a problem.
Therefore, it is preferable that the number of pixels to be
inserted be as small as possible. In addition, in order to prevent
the expansion of the width of an image in the sub-scanning
direction by the pixel insertion process, it is possible to perform
a thinning out process. For example, preferably, the insertion
process and the thinning out process are alternately performed by
the same number of times such that the insertion process is
performed on the first 500 lines and then the thinning out process
is performed on the next 500 lines. In this case, the pattern of
density variation is halved. In addition, the insertion process and
the thinning out process may be performed, for example, at a ratio
of 2 to 1.
[0067] Next, a structure for realizing the bad image quality
correcting method will be described.
[0068] FIG. 10 is a block diagram illustrating a structure of a
control unit 31 for executing the above-mentioned bad image quality
correcting process. The control unit 31 includes, for example, an
image data generating unit 51 for converting an input image into
image data peculiar to the image forming apparatus, a screen
processing unit 52 for performing a screen process on the image
output from the image data generating unit 51, and a registration
correction processing unit 53 for performing various processes,
such as skew correction, magnification process, and correction for
the screen. In addition, the control unit 31 further includes an
image formation instructing unit 54 for outputting image
information to the ROSs (an ROS for Y, an ROS for M, an ROS for C,
and an ROS for K) of the exposure device 13 corresponding to the
image forming units 10Y, 10M, 10C, and 10K for forming images
having respective colors Y, M, C, and K. Further, the control unit
31 still further includes a dot pattern storing unit 55 stored with
dot pattern information on the respective colors Y, M, C, and K or
on every object. Furthermore, the control unit 31 yet further
includes a registration detection processing unit 56 for detecting
the skew or misalignment of each color with respect to, for
example, black, which is a reference color, in the sub-scanning
direction using, for example, the color misalignment sensor 32 and
a registration correction value calculating unit 57 for calculating
a registration correction value when an image address in a header
is changed and output.
[0069] The image data generating unit 51 converts the image data
output from a personal computer or IIT, such as a page describing
language or bitmap data, into image data peculiar to the image
forming apparatus. The image data generated by the image data
generating unit 51 may be represented by 600 dpi (8 bits)+Tag (4
bits) by multi-valued data or may be represented by 600 dpi (1 bit)
or 1200 dpi (1 bit) by binary data. The screen processing unit 52
respectively performs a suitable process on a specific color and a
specific object (for example, photographs and characters are
separately processed), and then outputs, for example, image data of
2400 dpi (1 bit). In the screen processing unit 52, a text/image
separating (T/I separating) process is performed, and a dot pattern
is read from the dot pattern storing unit 55. The registration
detection processing unit 56 grasps periodic characteristics of
exposure of the ROSs (the ROS for Y, the ROS for M, the ROS for C,
and the ROS for K) of the exposure devices 13 corresponding to the
respective image forming units 10Y, 10M, 10C, and 10K and then
stores them therein. The registration correction value calculating
unit 57 calculates the insertion positions of the pixels or the
deviation amount of image output timing for every line for the bad
image quality correcting method. The registration correction
processing unit 53 performs the insertion process of the pixels or
the deviation process of image output timing for every line for the
bad image quality correcting method, using the registration
correction value calculated by the registration correction value
calculating unit 57. Further, the image data generating unit 51 is
provided in the control unit of the image forming apparatus, but
may be arranged in an external control unit provided in other
apparatuses other than the image forming apparatus.
[0070] Next, a process executed by the control unit 31 will be
described.
[0071] FIG. 11 is a flow chart illustrating the process executed by
the control unit 31 shown in FIG. 10. When an image output request
is received (step 101), the screen processing unit 52 of the
control unit 31 performs object determination, such as the
recognition of a text or image, on the image data output from the
image data generating unit 51 (step 102). In the screen processing
unit 52, a predetermined dot pattern is read from the dot pattern
storing unit 55 (step 103), based on the color of the image data
and the object determination, and a screen process is then
performed (step 104).
[0072] The registration correction processing unit 53 determines
whether skew correction should be performed or not (step 105).
Whether the skew correction should be performed or not is
determined, for example, based on the detection result of a pattern
for controlling color misalignment acquired by the color
misalignment sensor 32. For example, the skew correction may be
performed at the time of color registration. More specifically,
color registration may be performed by executing the skew
correction on other colors on the basis of one color, such as black
(K). When it is determined that it is not necessary to perform the
skew correction in step 105, the process proceeds to step 110. On
the other hand, when it is determined that it is necessary to
perform the skew correction in step 105, for example, a skew
correction process shown in FIG. 14B is performed (step 106).
[0073] Thereafter, in the registration correction processing unit
53, the fluctuation period of the scanning line is acquired based
on the periodic characteristic of exposure (step 107). When the
surface emitting laser device 40 shown in FIG. 2 is used, the
periodic characteristic of exposure may be caused by the deviation
the physical position of each laser diode 41. In addition, as shown
in FIG. 4A, the cause may also be the positional deviation of the
surface emitting laser device 40. Further, the cause may be a
variation in exposure amount caused by a difference in reflectance
between light components emitted from the respective laser diodes
41 in their optical paths. The registration correction processing
unit 53 determines whether the periodic characteristic of exposure
(scanning line) generated by these causes is synchronous with the
period of the screen determined by the screen processing unit 52
(step 108). When they are asynchronous with each other, the process
proceeds to step 110. When they are synchronous with each other,
the pixel insertion process is performed as shown in FIGS. 6B and
8B (step 109). In this pixel insertion process, if necessary, the
pixel thinning out process can be jointly performed. After the
pixel insertion process is completed in this way, image data is
output from the image formation instructing unit 54 to an IOT
(Image Output Terminal) (step 110).
[0074] As such, according to the present embodiment, in the
multi-beams emitted from, for example, the surface emitting laser
device 40 shown in FIG. 2, it is possible to correct a periodic
defect in image quality generated by the synchronization between
the periodicity of characteristics of the respective laser diodes
(LD) 41 and the alignment period of image data. When they are
asynchronous with each other, the problem of the periodic defect in
image quality does not arise. However, for example, when the
periodicity of a multi-beam characteristic is 4 and the image data
has a periodicity of 4, a defect in image quality easily occurs. An
example in which the image data has the periodicity of 4 includes a
case in which resolution is converted from binary data of 600 dpi
to a binary image of 2400 dpi. In addition, for example, the screen
of 212 lpi (line per inch) as shown in FIG. 12A has the periodicity
of 8 pixels or 16 pixels in the sub-scanning direction. Further, in
the surface emitting laser device 40 shown in FIG. 2, the position
of exposure is changed due to the periodicity of 4 in the
arrangement of the laser diodes (LDs) 41. However, in a case of the
periodicity of 32 lines formed by one scanning, or in a case in
which double exposure is preformed with the 16 lines overlapped,
period of the 16 lines may be a problem.
[0075] Meanwhile, FIG. 12B shows a screen not requiring the process
according to the present embodiment. A screen of 185 lpi shown in
FIG. 12B has periodic characteristics of 5 pixels and 12 pixels in
the sub-scanning direction. However, in the periodic
characteristics, the positions of dots are asynchronous with the
periodicity of 4 of the multi-beam, and thus the above-mentioned
defect does not occur. That is, odd numbers and even numbers are
present, and different laser diodes (LDs) 41 are used corresponding
to pixel positions. Therefore, it is not necessary to perform the
process according to the present embodiment to such a screen. In
step 108 shown in FIG. 11, in a case of such an image, the pixel
insertion process is omitted, and the process proceeds to step 110.
That is, in order to prevent the unnecessary process in the present
embodiment, whether to perform the pixel insertion process may be
determined according to image data. For example, determination may
be made such that the pixel insertion process is performed for a
binary image of 600 dpi or the screen shown in FIG. 12A, and such
that the pixel insertion process is not performed for the screen
shown in FIG. 12B. As described above, the screen is selected
corresponding to an object. Therefore, for example, when tag data
is added to every object, it is possible to perform the process
corresponding to the screen of multi-valued data, using the tag
data corresponding to the object. In addition, different processes
can be performed according to whether multi-valued data is used or
binary data is used or whether binary data of 600 dpi is used or
not.
[0076] Further, as shown in the flow chart of FIG. 11, a
precondition of the bad image quality correcting method according
to the present embodiment is to perform the skew correction shown
in step 106. For example, as the bad image quality correcting
method performed according to whether the skew correction is
executed, the following method can be used: the screen having the
periodicity of 4 is set to a specific color, such as black (K); it
is confirmed whether to perform the skew correction of black (K),
and then the process according to the present embodiment is
performed. In addition, another method can be used in which the
skew correction is not performed on black (K), which is the
reference of the skew correction, but performed on other colors,
and then the process according to the present embodiment is
performed. Further, it is preferable to avoid performing the skew
correction on black (K) as far as circumstances permit.
[0077] Furthermore, when the bad image quality correcting method
according to the present embodiment is performed on only one
specific color, for example, when the pixel insertion process is
performed on only black (K) at 500 intervals, a black (K) image is
enlarged in the sub-scanning direction by 0.2%. In addition, when
the pixel insertion process is not performed on three other colors
since it is determined that the process is not needed, the
magnification deviation between K and three colors (color
registration deviation in which head portions coincide with each
other, but the farther it is close to the tail of an image, the
larger the deviation is) occurs in the sub-scanning direction. In
order to prevent the magnification deviation, the pixel insertion
process is performed on one of four colors Y, M, C, and K. Then,
even when it is considered that the pixel insertion process is
unnecessary for improving the image defect, it is preferable to
perform the pixel insertion process for preventing color
misalignment. In this case, it is effective to set the positions of
pixels to be inserted to correspond to the screen shapes of the
respective colors.
[0078] Further, the above-mentioned skew correction is to correct
the inclination deviation of straight lines extending to the entire
width of an image in the main scanning direction. However, the
present embodiment can be applied to another process of outputting
an image having a local step difference. That is, the present
embodiment can be applied to correct the misalignment of a
non-linear image, that is, to correct the curvature (a so-called
bow) of an image by changing a step difference in the main scanning
direction or by changing the direction of the step difference.
[0079] As described above, according to the present embodiment, it
is possible to reduce defects in image quality caused by the
periodic variation of exposure position by the multi-beam and
caused by the periodicity of image data to be drawn, by correcting
defects in image quality in a manner such as inserting a pixel into
a predetermined pixel location corresponding to the original image
and by shifting it in the sub-scanning direction. Further, the
present embodiment can also be used to adjust the position of an
image. For example, the present embodiment can be used to
effectively correct a defect in monochromatic image quality
generated when selectively switching the laser diodes (LDs) 41 used
for outputting an image in the multi-beam, such as the surface
emitting laser device 40. As such, the process according to the
present embodiment can be applied to a black-and-white image
forming apparatus in addition to the color image forming
apparatus.
[0080] Furthermore, in the above-mentioned examples, the pixel
insertion process shown in step 109 of FIG. 11 is used as the image
shifting process in the sub-scanning direction. However, it is
possible to generate the variation of density in the sub-scanning
direction and to make periodicity in the main scanning direction
inconspicuous by performing the pixel thinning out process, instead
of the pixel insertion process.
[0081] Moreover, in the present embodiment, the image forming
apparatus adopting an electrophotographic method has been
described. However, the bad image quality correcting method
according to the present embodiment can also be applied to an image
forming apparatus adopting an inkjet method. When a plurality of
nozzles, which is printing sources, is provided instead of the
scanning of the multi-beam, an image shift process is performed in
the sub-scanning direction, which is the moving direction of an
image carrier (for example, a sheet of paper), by a printing unit
for printing an image on the image carrier (for example, a sheet of
paper) by the collective scanning of the plurality of nozzles. In
this way, it is possible to obtain the same effects as those in the
electrophotographic method. However, when the present embodiment is
applied to an electrophotographic method using the multi-beam ROS,
it is possible to obtain remarkable effects, which is not obtained
by the inkjet method. For example, it is possible to correct
periodic characteristics caused by the difference of a reflective
index in an optical path or the difference between the contact
positions of a rotary polygon mirror in the respective LDs. In
addition, for example, even when the characteristics of the
plurality of LDs are separately changed by the temperature
variation and even when refractive indexes are different from each
other by a difference in wavelength, it is possible to perform
correction. In this way, when the present embodiment is applied to
the image forming apparatus using the multi-beam ROS, it is
possible to obtain greater effects.
* * * * *